Decomposition of fluorine containing compounds

ABSTRACT

A process for the decomposition and removal of one or more fluorine containing compounds from a first gaseous mixture comprising the one or more fluorine containing compounds and water vapour, which process comprises the stages of: (i) contacting the first gaseous mixture with a catalyst comprising an aluminium based material to produce a second gaseous mixture comprising hydrogen fluoride and carbon oxides; and (ii) removing the hydrogen fluoride from the second gaseous mixture to produce a third gaseous mixture, which is substantially free of hydrogen fluoride.

[0001] This invention relates to a catalytic abatement process fordecomposing one or more fluorine containing compounds in an aqueousgaseous mixture. Fluorine containing compounds, such ashydrofluorocarbons and perfluorocarbons, are generated as by-products ina number of different industrial processes.

[0002] For example, in the semiconductor industry perfluorocarbons areused as cleaning and etching agents in the wafer fabrication process.Generally, the perfluorocarbons are fragmented to produce fluorinespecies that etch the wafer surface, creating fluorine containingby-products such as SiF₄. However, only about 20 to 30% of theperfluorocarbon gas is actually used in the process. Theperfluorocarbons that are not used in the process are often emitted intothe atmosphere.

[0003] The aluminium smelting and processing industry also generatesvast quantities of perfluorocarbons these processes produce significantamounts of perfluorocarbons, which are emitted into the atmosphere. Thespecies predominantly formed are tetrafluoromethane and perfluoroethane.

[0004] Perfluorocarbons have been shown to contribute to global warming.Emissions of these gases into the atmosphere should, therefore, beavoided. One way of achieving this is to destroy the perfluorocarbons soas to produce products that are more environmentally friendly.

[0005] Several methods for the recovery and abatement ofperfluorocarbons are known. These methods generally comprise combustion,catalytic and plasma based technologies. Recovery techniques generallycomprise solvation, membrane and cryogenic distillation processes.Processes for the recovery of perfluorocarbons are, however, often noteconomically viable as the exhaust gas streams are typically dilute andtypically only comprise from 2 to 3% perfluorocarbons.

[0006] U.S. Pat. No. 6,077,482 describes a process for decomposingorganohalogen compounds such as chlorofluorocarbons. A catalystcomprising titania and tungsten oxide is contacted with theorganohalogen compounds at a temperature of from 200 to 500° C.

[0007] JP-A-10192653 describes a process in which a gas streamcontaining at least one compound containing a fluorine atom and at leasttwo carbon atoms is contacted with a catalyst containing alumina,titania, silica and zirconia in the presence of steam at a temperatureof from 204 to 427° C. The catalyst activity was found to decrease overtime due to the formation of aluminium fluoride.

[0008] In both these examples a wet alkaline scrubber or an alkalinefilter is used to remove hydrogen fluoride by-products at the end of theprocess.

[0009] There are, however, a number of disadvantages associated withusing wet scrubbing systems. For example, corrosion of ducting andinstruments associated with the process occurs readily. Ducts alsobecome blocked because silica forms within them. The handling of aqueoushydrogen fluoride also has a number of problems associated with it, suchas safety, neutralisation and disposal. It is also necessary to ensurethat heavy metals such as copper and tungsten are removed from theliquors of the aqueous scrubbing systems as they are also harmful to theenvironment.

[0010] The use of alumina as a catalyst to hydrolyse perfluorocarbonshas also been investigated. Alumina was found to be ineffective as acatalyst because the hydrolysis reaction is inhibited due to theconversion of alumina to aluminium trifluoride (see Catalytic control ofemissions during semiconductor manufacture, Brown R., Rossin J. A., SIASemicon west 2000). Aluminium trifluoride is formed by reaction ofalumina with hydrogen fluoride produced in the hydrolysis reaction.

[0011] Thus, there is a need for an improved process for thedecomposition of perfluorocarbons in gaseous product streams which issimple, effective and industrially applicable.

[0012] The present invention provides a catalytic abatement process fordecomposing one or more fluorine containing compounds in a gaseousmixture containing water vapour.

[0013] In the process of the present invention, firstly fluorinecontaining species in the gaseous mixture are hydrolysed and secondlyhydrogen fluoride and other fluorine containing species are removed.

[0014] The process of the present invention is a two stage hydro-thermaltreatment process for a stream comprising one or more fluorinecontaining compounds. In the first stage the fluorine containingcompound(s) are hydrolysed using a catalyst comprising an aluminiumbased material, thus producing hydrogen fluoride and carbon oxides. Inthe second stage hydrogen fluoride is removed from the product of thefirst stage. This second stage may be performed using a solid comprisingan aluminium based material, such as an alumina, hydrated aluminiumoxide or aluminium hydroxide containing material, as a hydrogen fluorideabsorbent which may be referred to or termed the second stage catalyst.

[0015] The two stage process provides a high efficiency of decompositionof the fluorine containing compounds, where destruction efficiencies ofgreater than 99% may be achieved. Levels of hydrogen fluoride emissionare negligible. Typically, the absorption efficiency is greater than99%. Thus, the process of the invention removes the need for aqueousscrubbing post treatments.

[0016] In a preferred embodiment of the invention, the two stage processof the invention is repeated. In other words, in this embodiment theprocess comprises a hydrolysis step then an absorption step which arefollowed by a further hydrolysis step and absorption step. The two stageprocess may be repeated as many times as necessary.

[0017] In another embodiment of the invention, at least part of thegaseous product of the second stage (ii) of the process is recycled tothe first stage (i).

[0018] In another embodiment, the perfluorocarbon concentration in thefeed stream can be concentrated prior to being fed to the two stageprocess. This allows the volume of gas passing through the process to bereduced, providing smaller process reactors to be used at a higherenergy efficiency. A range of technologies may be used to concentratethe perfluorocarbon concentration in the feed stream including pressureswing or temperature swing adsorption/desorption, membranes,condensation and the solvent extraction of the perfluorocarbons from thegas stream to be treated. In the aluminium production industry, the useof zeolite adsorbents to trap and concentrate very dilute CF₄ from largegaseous vent streams may be particularly advantageous, when used withthe two stage process of the present invention.

[0019] The process of the present invention can be conducted at lowertemperatures that those used in alternative catalytic and combustionprocesses for perfluorocarbon abatement. The use of lower temperatureshelps to prevent the formation of NO_(x) by-products.

[0020] According to the present invention there is provided a processfor the decomposition and removal of one or more fluorine containingcompounds from a first gaseous mixture comprising the one or morefluorine containing compounds and water vapour, which process comprisesthe stages of (i) contacting the first gaseous mixture with a catalystcomprising an aluminium based material to produce a second gaseousmixture comprising hydrogen fluoride and carbon oxides; and (ii)removing the hydrogen fluoride from the second gaseous mixture toproduce a third gaseous mixture, which is substantially free of hydrogenfluoride.

[0021] Stage (i) of the process is a hydrolysis reaction. It is,therefore, essential that the first gaseous mixture comprises watervapour. Typically, the number of hydrogen atoms provided by the water isat least equal to the total number of fluorine atoms provided by thefluorine containing compounds, i.e. at least a stoicheiometric level ofwater to convert all fluorine containing compounds to oxides andhydrogen fluoride.

[0022] Preferably, the water level fed to the hydrolyser in stage (i) ofthe process should be sufficient to fully hydrolyse 1 to 100 times thequantity of fluorine containing compounds in the first gaseous mixture.More preferably, the level of excess water should be 1.5 to 40 times,most preferably 2 to 10 times, more than the stoichiometric requirementfor full hydrolysis of the fluorine containing compounds in the firstgaseous mixture.

[0023] The fluorine containing compounds which may be decomposed usingthe process of the present invention include hydrofluorocarbons andperfluorocarbons, which may be saturated or unsaturated. The fluorinecontaining compounds may also contain other heteroatoms such aschlorine.

[0024] By the term “hydrofluorocarbon” we mean compounds that containonly carbon, hydrogen and fluorine atoms in their structure. By the term“perfluorocarbons” we mean compounds that contain only carbon andfluorine in their structure.

[0025] Preferably, the fluorine containing compounds such ashydrofluorocarbons and perfluorocarbons comprise from one to twelvecarbon atoms, more preferably from one to six carbon atoms and stillmore preferably from one to four carbon atoms. Typically, the fluorinecontaining compounds are straight or branched chain or cyclic organiccompounds.

[0026] Particularly preferred hydrofluorocarbons and perfluorocarbonsinclude tetrafluoromethane, trifluoromethane, perfluoroethane,perfluoropropane, octafluorobutane (both isomers), pentafluoroethane(R125), difluoromethane (R32) and tetrafluoroethane (R134a). Still morepreferably, the fluorine containing compound is tetrafluoromethane orperfluoroethane.

[0027] Suitable catalysts comprising an aluminium based material for usein the first stage of the process of the invention include aluminiumoxide (alumina), hydrated aluminium oxide, aluminium hydroxide,aluminium oxyfluoride and aluminium fluoride.

[0028] The operating catalyst typically has a surface area of 5 m²/g orgreater, preferably 10 m²/g or greater and more preferably 20 m²/g orgreater. It is most preferred that the catalyst has a surface area of 40m²/g or greater. As the hydrolysis conditions cause a progressive lossof surface area, the preferred catalysts comprising an aluminium basedmaterial have an initial surface area of 5 m²/g or greater, morepreferably 50 m²/g or greater. The preferred catalysts comprising analuminium based material also have a high thermal stability in the hightemperature hydrolysis conditions and in the presence of water andhydrogen fluoride gases.

[0029] Suitable absorbents comprising an aluminium based material foruse in the second stage of the process of the invention includealuminium oxide (alumina), hydrated aluminium oxide, aluminium hydroxideand aluminium oxyfluoride. Preferably the absorbent used in the secondstage has a high surface area. The use of absorbents having a surfacearea of at least 50 m²/g is preferred. For example, alumina having asurface area of 200 m²/g or greater can be used.

[0030] In a preferred embodiment of the process, the same aluminiumbased material may be used in the first stage and the second stage. Inthis case, the same aluminium based material may be used in the secondstage before it is used in the first stage. Thus, it is not essentialfor the aluminium based material used in the first stage to be “fresh”ie, it may be contaminated to some extent with, for example, hydrogenfluoride, or be partly converted to aluminium fluoride.

[0031] Preferably, the aluminium based materials used possess sufficientthermal stability to retain a surface area of 20 m²/g or greater in thestage (i) hydrolysis reactor and thus provide sustained high levels offluorocarbon destruction.

[0032] The aluminium based material used in either stage (i) or stage(ii) may be subjected to pre-treatment prior to use. For example, thealuminium based material may be pre-treated by being subjected tothermal or hydrothermal treatments. Hydrothermal treatment is especiallyfavourable for aluminium based materials with high fluoride contents, ashigh temperature steam treatments increase the surface oxide contentsand the associated fluorochemical hydrolysis activity of the aluminiumbased material.

[0033] Steam increases the oxide level on the aluminium based materialand hence increases the activity of the material as a catalyst. Forexample, steam passed over a catalyst comprising an aluminium basedmaterial introduces oxides into the structure. Thus, stream treatmentcan be considered to be an activation step that increases theperformance of the catalyst. Steam can also be used to regenerate apreviously used catalyst. Additionally, steam can be used to recover aheavily fluorinated catalyst and to recover the catalyst from an upsetcondition.

[0034] During steam treatment water vapour is passed through thecatalyst bed. This is typically carried out at a temperature of greaterthan 500° C., preferably from 600 to 800° C. The steam treatmenttypically takes from 1 to 60 minutes, preferably about 10 minutes, at700 to 750° C.

[0035] Stage (i) of the process of the invention is typically conductedat a temperature of 450° C. or more, and preferably at a temperature offrom 500 to 1000° C., more preferably at a temperature of from 650 to800° C., for example at about 700 to 750° C.

[0036] Stage (i) of the process of the invention can be carried out atatmospheric, subatmospheric or superatomospheric pressure. Preferably,stage (i) is carried out at atmospheric pressure or at a pressure alittle above or below atmospheric pressure.

[0037] Stage (i) of the process produces hydrogen fluoride and carbonoxides. Operation of the unit at sub-atmospheric pressure reduces therisk of gaseous emissions from the reaction zone, which contains toxichydrogen fluoride and carbon monoxide gases.

[0038] Typically, stage (i) of the process of the present inventionprovides a 99% conversion of the perfluorocarbons andhydrofluorocarbons.

[0039] High proportions of carbon monoxide may be formed in thehydrolysis of hydrofluorocarbons and higher molecular weightperfluorocarbons. Air may be added to the first gaseous mixture in orderto reduce the level of carbon monoxide formed. The air oxidises thecarbon monoxide to carbon dioxide, which can be released into theatmosphere without further treatment.

[0040] The efficacy of stage (i) of the process may be increased by theaddition of additives to the catalyst comprising an aluminium basedmaterial. Suitable additives include the metals of Groups 4 to 14 of theperiodic table of elements and compounds comprising one or more metalsfrom these groups. These improve alumina hydrothermal stability and thussurface area retention in the hydrolysis reactor of stage (i). Zincoxide is an example of a useful alumina stabiliser for the process ofthe invention. Redox metals within the above Groups also increase thecarbon monoxide oxidation rates in the process. Iron or compounds ofiron are preferred. For example, oxides, hydroxides or hydrated oxidesof iron may be mixed with the catalyst comprising an aluminium basedmaterial.

[0041] The reference to Groups 4 to 14 of the periodic table of theelements refers to the new IUPAC version of the periodic table ofelements.

[0042] For example, zinc or a zinc compound such as zinc oxide can beadded to the catalyst in order to improve hydrolysis. For example, theuse of zinc or a zinc containing compound may increase surface areastability and thus increase the length of time for which the catalyst isactive. Iron or an iron containing compound can be added to increasecarbon monoxide oxidation rates. Suitable iron containing compoundsinclude iron oxides, such as Fe₂O₃.

[0043] A particularly preferred catalyst comprises zinc and Fe₂O₃ on analuminium oxyfluoride support. Another preferred catalyst comprises zincon an alumina support.

[0044] Another particularly preferred catalyst is an aluminium basedcatalyst comprising an aluminium oxide, hydrated aluminium oxide,aluminium hydroxide, aluminium oxyfluoride or aluminium fluoride, with asurface area of 50 m²/g or greater and a pore volume of 0.3 cc/g orgreater. The surface area of the catalyst is located in the pores, whichhave a diameter of 40 Å or greater, preferably 50 Å or greater.Preferred alumina based catalysts have a combined alkali and alkalineearth metal content of less than 1% w/w, more preferably less than 0.5%w/w.

[0045] Water vapour can be injected into the first gaseous mixture inorder to promote hydrolysis.

[0046] In stage (ii) of the process of the invention, the hydrogenfluoride can be removed from the second gaseous mixture by any suitablemethod. For example hydrogen fluoride can be removed using methods wellknown in the art such as water scrubbing, aqueous alkali scrubbing,reaction with alkaline metal earth metal oxides and absorption into anamine hydrohalide, a glycol or sulphuric acid.

[0047] In a preferred aspect of the present invention, hydrogen fluoridemay be removed from the second gaseous mixture by contacting it with anabsorbent comprising an aluminium based material, such as aluminiumoxide, hydrated aluminium oxide, aluminium hydroxide or aluminiumoxyfluoride. Alumina based materials with low silica contents arepreferred, as such materials limit the amount of volatile fluorinecontaining compounds of silicon entering the process vent stream.

[0048] The aluminium based material used in the second stage may besubjected to pre-treatment prior to use. For example, steam treatment,as described above, may be used.

[0049] Preferably, the second gaseous mixture is contacted with theabsorbent comprising an aluminium based material at a temperature of500° C. or less, more preferably 400° C. or less and most preferably ata temperature of from 275 to 375° C., for example, 350° C. It ispreferred that stage (ii) of the process is conducted at a temperaturebelow the temperature at which stage (i) is conducted.

[0050] The second stage of the process of the invention may be carriedout at atmospheric, subatmospheric or superatmospheric pressure.Preferably, the second stage is carried out at atmospheric pressure orat a pressure a little above or below atmospheric pressure. Preferably,the second stage of the process of the invention is carried out at thesame pressure or a similar pressure to the first stage of the process.

[0051] The same aluminium based material may be used in both stages (i)and (ii) of the process of the invention. It is preferred that thealuminium based material used in stage (ii) is also used as thealuminium based material for stage (i). This can be achieved by using amoving bed to move the aluminium based material from the reaction zonefor stage (ii) to the reaction zone for stage (i). In this example, thebed moves in a direction counter-current to the gas flow.

[0052] In an alternative embodiment, in stage (ii) of the process thehydrogen fluoride may be removed from the second mixture by absorbingthe hydrogen fluoride using a conventional alkali or water scrubber.

[0053] The reaction residence time for each stage of the process of theinvention is preferably up to about 40 seconds, preferably from 0.1 to10 seconds, and more preferably from 0.2 to 5 seconds under reactionconditions.

[0054] The process of the invention may comprise a third stage (iii) inwhich materials other than fluorine containing organic entities can beremoved from the first gaseous mixture in a pre-treatment step beforethe first gaseous mixture is subjected to stage (i). For example, otherfluorine containing compounds such as non-organic fluorine containingcompounds, for example SiF₄ and WF₆ can be removed. These materials canbe removed using methods that are conventional in the art such as by theuse of water scrubbers.

[0055] Alternatively, inorganic fluorine containing compounds can beremoved from a gaseous mixture comprising these compounds and one ormore organic fluorine containing compounds by passing the gaseousmixture over an aluminium based material. Suitable aluminium basedmaterials include those described above, for example, aluminium oxide(alumina), hydrated aluminium oxide, aluminium hydroxide, aluminiumoxyfluoride and aluminium fluoride. These aluminium based materials maycontain additives as described above.

[0056] The inorganic fluorine containing compounds may be removed fromthe first gaseous mixture by hydrolysis to an oxide or oxyfluoride,where the inorganic compound is deposited on a solid comprisingaluminium oxide, hydrated aluminium oxide, aluminium hydroxide,aluminium oxyfluoride or aluminium fluoride. The inorganic fluorinecontaining compound may be deposited on the aluminium based materialused in stage (i). The inorganic fluorine containing compound istypically deposited on the aluminium based material at a temperaturebelow 800° C., for example from 0 to 500° C., and a lower temperaturethan that at which stage (i) is conducted.

[0057] The stage for removing the inorganic fluorine containingcompounds is preferably used as a pre-treatment step in combination withthe two stage process of the present invention. Thus, the presentinvention also provides a three stage process comprising pre-treatmentof the first gaseous mixture to remove inorganic fluorine containingcompounds followed by the two stage process described above.

[0058] Preferably, the stage for removing inorganic fluorine containingcompounds is conducted at a temperature of from 100 to 800° C., morepreferably from 200 to 500° C. When this process is carried out incombination with the two stage process of the invention the temperatureat which this process is carried out is typically lower than that usedin the first stage (i) of the two stage process.

[0059] The reaction residence time of stage (iii) is preferably from 0.1to 30 seconds.

[0060] The stage for removing inorganic fluorine containing compounds istypically carried out at atmospheric, subatmospheric or superatmosphericpressure. Preferably atmospheric pressure or a pressure a little aboveor below atmospheric pressure is used. If this stage is carried out incombination with the two stage process described above this stage (iii)is preferably carried out at the same or similar pressure as used in thefirst stage of the two stage process, more preferably at the same orsimilar pressure as used in both stages of the two stage process.

[0061] When the stage (iii) for removing inorganic fluorine containingcompounds is carried out in combination with the two stage processdescribed above the aluminium based material used in stage (iii) ispreferably the same as that used in one or both of the other two stages.Most preferably, the same aluminium based material is used in all threestages. In this case, spent aluminium based material from stages (i) and(ii) can be used in the process for removing non-organic fluorinecontaining compounds.

[0062] It is preferred that at least a proportion of the gas vented fromstage (ii) of the process is recycled to the reaction zone for stage (i)and/or stage (iii).

[0063] The apparatus that may be used to carry out the processes of thepresent invention may employ a moving bed of aluminium based material,which moves counter current to the gas flow. Preferably the moving bedwill pass through the reaction zones for both stages of the two stageprocess. If the stage for removing inorganic fluorine containingcompounds is also used, the moving bed will preferably also pass throughthe reaction zone for this stage. Thus, in a preferred embodiment thealuminium based material initially passes through the reaction zone forthe second stage (ii) of the process and then through the reaction zonefor the first stage (i) of the process and then, optionally, through thereaction zone for the stage (iii) for removing inorganic fluorinecontaining compounds.

[0064] It will be appreciated that when a three stage process is used,stages (i) and (ii) may be repeated as described above. Also, when athree stage process is used the gaseous product from stage (ii) may berecycled either into the reaction zone for stage (i) or into thereaction zone for stage (iii).

[0065] An example of a suitable apparatus in which the two stage processof the present invention may be conducted will now be described, by wayof non-limiting example, with reference to FIG. 1.

[0066]FIG. 1 shows an apparatus suitable for use in the process of thepresent invention. Using the apparatus of FIG. 1 the same aluminiumbased material is used for both stages (i) and (ii) of the process.

[0067] The apparatus (10) comprises first and second reaction zones,(12) and (14). A first gaseous mixture comprising one or more fluorinecontaining compounds and water vapour is fed into the first reactionzone (12) at inlet (16). This mixture passes through the first reactionzone (12) and then through the second reaction zone (14).

[0068] Whilst the first gaseous mixture is fed into the apparatus, acatalyst comprising an aluminium based material is fed into the secondreaction zone (14) at inlet (18). The direction in which the aluminiumbased material flows is opposite to that in which the first gaseousmixture flows. The aluminium based material passes through the secondreaction zone (14) and then the first reaction zone (12) and contactsthe first gaseous mixture.

[0069] As the first gaseous mixture passes through the two reactionzones, it contacts the aluminium based material so that fluorinecontaining species are decomposed and removed. In the first reactionzone the hydrofluorocarbons and/or perfluorocarbons are decomposed. Inthe second reaction zone, the hydrogen fluoride is removed. Gassubstantially free of fluorine containing species exits the apparatus(10) at exit (20). The spent aluminium based material exits theapparatus (10) at exit (22).

[0070] The spent aluminium based material, from the two stage process orthe process for removing non-organic fluorine containing compounds, mayeither be reprocessed or disposed of. It is especially preferred torecycle the aluminium based material when the catalytic abatementprocess is used in the aluminium industry where the recycled materialscan be fed into smelters.

[0071] The process is exemplified but not restricted by the followingexamples.

EXAMPLE 1

[0072] Stage (i) of the Process of the Invention

[0073] A hydrolysis catalyst comprising an aluminium based material wasprepared using alumina with a pore volume of greater than 0.3 cc/g and asurface area of greater than 50 m²/g in pores of greater than 50 Å. Thealumina was supplied by Engelhard and was coded Al-3996-R. The aluminahad a surface area of 200 m²/g and a pore volume of 0.75 cc/g. Thealumina was chosen to be low in alkali metal content, having a sodiumcontent of 0.01 % w/w.

[0074] The alumina catalyst thus formed was then used as a catalyst fora process corresponding to stage (i) of the process of the presentinvention.

[0075] The solid extrudate was crushed and sieved to generate aluminaparticles having a particle size of from 0.5 to 1.5 mm. The aluminaparticles were packed into an inconel reaction tube with an internaldiameter of 8 mm and located in a tube heater to provide a stage (i)reactor. The alumina had a packed bed density of 0.59 g/cc. A stream ofnitrogen was saturated in water at 20° C. and combined with a stream ofcarbon tetrafluoride to make a gas mixture containing 5000 parts permillion of carbon tetrafluoride by volume. The H₂O:CF₄ feed ratio wasestimated to be approximately 20000:5000 by volume, which was double thewater requirement for the stoichiometric hydrolysis reaction of CF₄ toCO₂.

[0076] The moist nitrogen and CF₄ gas mixture was passed over thealumina catalyst at atmospheric pressure, and the stage (i) reactor wasthen heated to 700° C. The contact time of the reaction gas mixtureunder the 700° C. reaction conditions was 4.4 seconds. After 12 hours ofreaction, the levels of CF₄, CO₂ and HF in the vent stream were measuredand the CF₄ hydrolysis efficiency was calculated to be greater than 90%.After the 12 hours of reaction, less that 5% of the HF hydrolysisproduct was absorbed on the stage (i) alumina catalyst. The catalyst inthe high temperature hydrolysis reactor was calculated to have a low HFabsorption efficiency with less than 10% of the aluminium oxide contentconverting to AlF₃.

EXAMPLE 2

[0077] Stage (ii) of the Process of the Invention

[0078] A second inconel reactor was charged with 0.5 to 1.5 mm particlesof Engelhard Al-3996-R alumina to provide a stage (ii) reactor. Thereactor was heated to 250° C. and connected to the vent gas line fromthe stage (i) reactor of Example 1. The level of HF in the gases ventedfrom the stage (ii) reactor was low, indicating that the HF absorptionefficiency was 99.4%, when using a contact time of 1.2 seconds. After 15hours of operation the HF absorber efficiency decreased sharply andafter 20 hours the absorbent failed to absorb additional HF. The aluminain the absorption stage was calculated to have a high HF absorptionefficiency with greater than 60% of the aluminium oxide contentconverting to AlF₃.

EXAMPLE 3

[0079] Stage (ii) of the Process of the Invention

[0080] The spent absorbent was discharged from the stage (ii) reactorand was replaced by an identical fresh alumina charge of EngelhardAl-3996-R alumina. The stage (ii) reactor was heated to 150° C. andconnected to the vent gas line from the stage (i) reactor operating at700° C., as described in Example 1. The CF₄ hydrolysis efficiency of thefirst stage reactor remained high, with above 90% of the CF₄ beingconverted to CO₂ and HF. The HF absorption efficiency of the stage (ii)alumina charge was found to be 99.4% over the first 10 hours ofreaction, when using a contact time of 1.4 seconds. After 13 hours ofoperation, the stage (ii) alumina bed was exhausted and failed to absorbsignificant additional levels of the HF produced in stage (i). Thealuminium oxide in stage (ii) had been 34% converted to AlF₃.

EXAMPLE 4

[0081] Stages (i) and (ii) of the Process of the Invention

[0082] The reactors used in Example 3 were emptied and the exhaustedabsorbent from the stage (ii) reactor was placed into the stage (i)hydrolysis reactor. The stage (ii) absorbent charge was replaced, usinga repeat charge of fresh Engelhard Al-3996-R alumina. The temperature ofthe stage (i) reactor was raised to 750° C. and the stage (ii) absorberwas heated to 250° C., whilst passing a water saturated nitrogen streamcontaining 2500 ppm by volume of CF₄. The H₂O:CF₄ feed ratio wasestimated to be approximately 20000:2500 by volume, which was four timesthe water requirement for the stoichiometric hydrolysis reaction of CF₄to CO₂. The contact times employed under reaction conditions were 2.1seconds in stage (i) and 4.2 seconds in stage (ii), where equal volumesof solid were charged to both stages of the process. After 100 hours ofoperation, the CF₄ destruction efficiency was maintained at greater than90%, using the cascaded spent alumina absorbent. The HF absorptionefficiency was found to be 99.6%, prior to exhaustion of the HFabsorption capacity. The stage (ii) charge of aluminium oxide was foundto convert 70% to AlF₃ in this study.

EXAMPLE 5

[0083] Stages (i) and (ii) of the Process of the Invention

[0084] Equal masses of the fresh alumina catalyst detailed in Example 1were charged to the two stages of the process. The stage (i) reactor wasthen heated to 750° C. and the stage (ii) reactor was heated to 350° C.,whilst passing a water saturated nitrogen stream containing 2500 ppm byvolume of CF₄, as described in Example 4. The contact time in the firsthydrolysis reaction stage was 2.1 seconds and the contact time in thesecond stage HF absorber was 3.5 seconds under reaction conditions. Thetwo stage CF₄ destruction process was operated until the second stagealumina absorbent was exhausted and the hydrolysis HF appeared in thevent from the second reaction stage. The two stage reactor system wasoperated for 92 hours and the CF₄ destruction efficiency was sustainedabove 99.5%. The HF absorbed by the two stage process was sufficient toconvert 66% of the second stage aluminium oxide to AlF₃.

EXAMPLE 6

[0085] Stages (i) and (ii) of the Process of the Invention

[0086] The two reactors used in Example 5 were emptied and the spentabsorbent was transferred to the stage (i) hydrolysis reactor. The stage(ii) absorber was recharged with fresh alumina as described in Example5. The stage (i) reactor was then heated to 750° C. and the stage (ii)reactor was heated to 400° C., whilst passing a water saturated nitrogenstream containing 2500 ppm by volume of CF₄, as described in Example 4.The contact time in the first hydrolysis reaction stage was 2.1 secondsand the contact time second stage HF absorber was 3.2 seconds underreaction conditions. The two stage reactor system was operated for 50hours and the CF₄ destruction efficiency was sustained above 99.0%. TheHF absorbed by the two stage process was sufficient to convert only 22%of the second stage aluminium oxide to AlF₃.

1. A process for the decomposition and removal of one or more fluorinecontaining compounds from a first gaseous mixture comprising the one ormore fluorine containing compounds and water vapour, which processcomprises the stages of: (i) contacting the first gaseous mixture with acatalyst comprising an aluminium based material to produce a secondgaseous mixture comprising hydrogen fluoride and carbon oxides; and (ii)contacting the second mixture with an absorbent comprising an aluminiumbased material to remove hydrogen fluoride from the second gaseousmixture and to produce a third gaseous mixture, which is substantiallyfree of hydrogen fluoride; wherein the aluminium based material used instage (i) has previously been used in stage (ii).
 2. A process accordingto claim 1, wherein at least one of the fluorine containing compounds isa hydrofluorocarbon or a perfluorocarbon.
 3. A process according toclaim 2, wherein the hydrofluorocarbon or perfluorocarbon has a carbonchain length of from one to four.
 4. A process according to claim 2 or3, wherein the hydrofluorocarbon or perfluorocarbon istetrafluoromethane, trifluoromethane, perfluoroethane, perfluoropropane,octafluorobutane, pentafluoroethane, difluoromethane ortetrafluoroethane.
 5. A process according to claim 4, wherein theperfluorocarbon is tetrafluoromethane or perfluoroethane.
 6. A processaccording to any one of the preceding claims, wherein the aluminiumbased material initially comprises aluminium oxide, hydrated aluminiumoxide, aluminium hydroxide or aluminium oxyfluoride.
 7. A processaccording to any one of the preceding claims, wherein stage (i) isconducted at a temperature in the range of from 500 to 1000° C.
 8. Aprocess according to any one of the preceding claims, wherein thealuminium based material comprises at least one metal or compound of ametal of Groups 4 to 14 of the periodic table.
 9. A process according toany one of the preceding claims, wherein stage (ii) is conducted at atemperature of from 100° C. to 500° C. and below the temperature ofstage (i).
 10. A process according to any one of the preceding claims,wherein a moving bed moves the aluminium based material from thereaction zone for stage (ii) to the reaction zone for stage (i) in adirection counter-current to the gas flow.
 11. A process according toany one of the preceding claims, wherein the gas residence time in eachstage of the process is from 0.1 to 40 seconds.
 12. A process accordingto any one of the preceding claims, further comprising stage (iii) inwhich at least one inorganic fluorine-containing compound is removedfrom the first gaseous mixture by deposition on a solid comprising analuminium based material.
 13. A process according to claim 12, whereinthe solid used in stage (iii) comprises aluminium oxide, hydratedaluminium oxide, aluminium hydroxide, aluminium oxyfluoride or aluminiumfluoride.
 14. A process according to claim 15 or 16, wherein stage (iii)is conducted at a temperature of from 0 to 800° C. and the reactionresidence time is from 0.1 to 30 seconds.
 15. A process according to anyone of the preceding claims, wherein the solid used in stage (iii) haspreviously been used in stage (i).
 16. A process according to claim 15,wherein stage (iii) is conducted at a temperature of from 0 to 500° C.and a lower temperature than that at which stage (i) is conducted.
 17. Aprocess according to claim 16, wherein a moving bed moves the aluminiumbased material from the reaction zone for stage (ii) to the reactionzone for stage (i) and then to the reaction zone for stage (iii).
 18. Aprocess according to any one of the preceding claims wherein thealuminium based material is activated or reactivated by treatment withsteam.
 19. A process according to any one of the preceding claims,wherein at least a proportion of the gas vented from stage (ii) isrecycled to the reaction zone for stage (i) and/or stage (iii).